Across:
1 Some havoc on Manitoulin (some 'havoC ON MANitoulin' = CON MAN) produced by fraudster (CON MAN),
4 Incitement to rebellion (SEDITION) in new (= anagram indicator) editions (editions = SEDITION),
9 Serves up (= reverse indicator) starters resting on tuna (starters ‘Resting On Tuna’ = rot = -TOR) fish (cod = DOC-) (DOCTOR) for sister’s colleague (DOCTOR),
10 Writing (from the 3Rs = -R) to James the First (‘James’ the first = J-) from university (-U-) surrounded by Hawaiian Missionaries (Hawaiian Missionaries postage stamp = STA- -MP)? (STAR JUMP) To achieve this, you’ll have to work out! (STAR JUMP),
12 One hoping to improve another’s standing perhaps (CANVASSER) is known to have walked the wards (CANVASSER used to walk political wards/constituencies),
13 Category (GENRE) of the odd green brier (the odd letters in ‘GrEeN bRiEr’ = GENRE),
14 Make Istanbul (make Istanbul) nervous (= anagram indicator) (make Istanbul = UNMISTAKABLE) beyond a doubt (UNMISTAKABLE),
18 Monitor (SCREEN-) bookmaker (-WRITER) (SCREENWRITER) who produced The Script? (SCREENWRITER),
21 Just (RIGHT) remove fine art from freight train (remove ‘fine art’ from ‘freight train’ = rghti = RIGHT),
22 Moving arms (GUNRUNNER) – it’s key (music key = G-) for a French (‘a’ French = -UN-) athlete (-RUNNER) (GUNRUNNER),
24 Hopefully optimistic (SANGUINE) genius (genius) wrongly (= anagram indicator) (genius = S-GUINE) welcomes article (-AN-) (SANGUINE),
25 Opens 11 down (opens ‘estate agency’ = ESTATE) with what’s left when one departs (ESTATE is left when a person departs/dies),
26 Do (do) set-up (= reversal indicator) (do = OD-) one of the ordinates (‘x and y ordinates’: -Y-) for ship (-SS-) at sea? (= anagram indicator) Yes (yes = -EYS) (ODYSSEYS) – expect old Greek travellers arriving on such like! (ODYSSEYS),
27 Parade (STREET) of various (= anagram indicator) trees (trees = STREE-) from the bottom of the forest (bottom of the ‘foresT’ = -T) (STREET).
Down:
1 Mussolini (-DUCE-), consumed by movement (cause), is not finished (e) ('cause' without 'e' = caus = CA-US) (CADUCEUS) with the messenger's staff (wand/staff carried by god Hermes/the messenger = CADUCEUS),
2 Steal (NICK -) someone’s identity (-NAME) (NICKNAME) or familiar moniker (NICKNAME),
3 Turns up (= reversal indicator), most of (e) the space (‘hole’ without ‘e’ = hol = -LOH-) taken up by non-drinkers (A-A) (ALOHA) is for greeting overseas party (ALOHA),
5 The key (ENTER key) to working out (training) is not having resistance (r) (‘training’ without ‘r’ -TAINING) (ENTERTAINING) to putting on a night for friends and family (ENTERTAINING),
6 Not even in uniform perhaps (IRREGULAR) and that’s doesn’t sound right (IRREGULAR),
7 One (I-) region in South America (Guiana) that’s doesn’t have one (i) (‘guIana’ without ‘i’ = -GUANA) (IGUANA) pet – the tropical type (IGUANA),
8 Serves up (= reversal indicator) bread (pan = NAP-) from the French (‘the’ French = -LES) (NAPLES) city on the Mediterranean (NAPLES),
11 20 down (‘agency’ = AGENCY) is behind vineyard (ESTATE) (ESTATE AGENCY) accommodating the homeless (ESTATE AGENCY),
15 Sue reacts (sue reacts) badly (= anagram indicator) (sue reacts = SECATEURS) to those employed in the plot (SECATEURS),
16 One of the Wise Guys (sage = S- AGE) getting to grips with way of speaking (tone = - TONE) (STONE AGE), for a time? (STONE AGE),
17 Park (P-) Run (run = -RU-N-) around most (e) of the country (Eire) (‘eirE’ without ‘e’ = eir) backed (= reverse indicator) (eir = -RIE-) for first time (first ‘Time’ = -T) (PRURIENT) by type that’s rather naughty (PRURIENT),
19 Old agricultural worker (serf) upsets (= reverse indicator) (serf = FRES-) Charlie (-C-) starting oil (starting ‘Oil’ = -O) (FRESCO) painting (FRESCO),
20 Closes 16 down (closes ‘Stone Age’ = AGE-) for NYC (NYC) boss (= anagram indicator) (NYC = -NCY) (AGENCY) of business providing a particular service (AGENCY),
23 In a state (UPSET) of shock? (UPSET).